Traditionally, combustible gas detectors used to detect the presence of combustible gases, such as those found in coal mines or manufacturing facilities, utilized a circuit configuration comprised of at least one sensing element. This sensing element was a wire having a catalytic coating sensitive to a gas. The sensing element was used as one of four legs of a Wheatstone bridge circuit. The other three legs consisted of two resistors and a compensator element. The compensator element was identical to the sensing element except that it did not bear a catalytic coating.
A current or voltage was applied to the bridge circuit to heat the surface of the catalytic coating affixed to the sensing element. Since the resistance values of the other three legs of the bridge were known, the resistance in the sensing element could be determined as the current or voltage was passed through the bridge.
When the sensing element was exposed to a combustible gas, such as methane, the catalytic coating would begin to burn the gas, increasing the temperature of the sensing element. As the temperature of the sensing element increased, the resistance of the element increased. Accordingly, the current or voltage passing through the element decreased. By comparing the resistance level of the sensing element to the resistance level of the compensator element, the presence of a combustible gas could be detected. Since a change in the gas concentration caused a change in the resistance of the sensing element, the quantity of the gas could be accurately determined by calibrating the change in resistance. This is the basic principle of operation of a catalytic combustible gas sensor.
Catalytic sensing elements can be, for example, a wire having a palladium bead coating (hereinafter referred to as a "pelement") or a wire filament that is coated with platinum. Sensing elements of the
type mentioned are disclosed in Baker, U.S. Pat. No. 3,092,799. The catalytic coating of a pelement provides a large number of active sites that promote combustion of the flammable gas.
Over a period of time, the sensitivity of the pelement tends to diminish. This reduced sensitivity most often occurs in instruments that are used intermittently. Intermittent use of the instrument causes the pelement to be heated when the instrument is turned on and then cooled when the instrument is shut off. When a high temperature in the pelement is not maintained, certain components found in the test gas do not burn completely. These components, known as inhibitors, tend to occlude, foul or poison the active sites found on the pelement surface. As these sites become poisoned, less test gas is burned on the pelement's surface, causing a reduction of surface temperature and decreased sensitivity.
The inhibitors can be burnt off the active sites by driving a high current through the pelement for a short period of time. It is the intent of this invention to provide a combustible gas detector having a circuit to enable a high current to be driven through the catalytic sensing element for a specific period of time in order to maintain the instrument's accuracy and reliability.